Recovery of naphthene hydrocarbons



Patented July 12, 1949 RECOVERY oF NAPHTHENE HYDRooARBoNs AdalbertFarkas and Arthur F. Stribley, Jr., Long Beach, Calif., assignors toUnion Oil Company of California, Los Angeles, Calif., a corporation ofCalifornia Application March 5, 1945, Serial No. 581,162

(Cl. 26o-666) Claims.

The invention relates to a process and apparatus for the recovery ofna'phthene hydrocarbons from petroleum hydrocarbon fractions. Moreparticularly the invention contemplates preparation of pure naphthenehydrocarbons from petroleum fractions or from mixtures of isomericnaphthene hydrocarbons.

The process of preparing pure components from complex hydrocarbonfractions by isolating said components from the fraction is well known.There are many methods by which this type of separation has beenaccomplished as for example fractional distillation, azeotropicdistillation, extractive distillation, solvent extraction, selectiveadsorption, acid treatment and the like. O'f these processes, fractionaldistillation is probably the most generally employed for the separationof naphthene hydrocarbons from petroleum mixtures. However, in a largenumber of cases naphthenic and paraflinic hydrocarbons as found togetherin petroleum mixtures, have such close boiling points that theirseparation by distillation becomes impracticable. Further, because ofthe similarity in the properties of the naphthenic and paraiflnichydrocarbon no successful method has been found to date for effectingtheir separation by azeotroly; solvent extraction or the like. Theselatter processes depend upon variations of the physical properties ofthe components of a mixture in order to separate one or the other byeffecting a, change in the vapor pressure equilibria or by dependingupon a differential solubility of the components of the mixture in asolvent employed in solvent extraction. Very little success has resultedfrom experiments in the separation of the paraiiins and naphthenes by'az'eotropy' or solvent extraction because of 'this' similarity in theirproperties andthe lack of azeotrope farmers or solvent to selectivelychange their boiling points or to separate thernby selective solubility.

It is an Object of our invention therefore t0 recover naphthenichydrocarbons in substantially pure form from complex petroleumhydrocarbon ini'xtures in which they 'are contained.

It is a, further object of our invention to separate napnthenic andparaiiinic hydrocarbons from mixtures in 4which `they are contained. i

Another object of our invention vis directed to the Y.preparation ofcertain specific naphthene isomers in substantially pure form fromcomplex hydrocarbon mixtures.

It is a further object of our invention to prepare pure paraflinhydrocarbons by removal of naphthene hydrocarbons from mixtures thereof.

Other objects and advantages of our invention will become apparent tothose skilled in the art as the description thereof proceeds.

Briefly, we have found that by a suitable coinbination of fractional4distillation and catalytic isomerization it is possible to separatenaphthene hydrocarbons from petroleum fractions contain= ing as wellparafn hydrocarbons. Specically the invention comprises in isolating anarrow boiling fraction containing naphthenic and paraflinichydrocarbons, which fraction may also contain other hydrocarbons,subjecting the fraction to a catalytic isomerization to effect theisomerization of the naphthene hydrocarbons in the fraction thusbringing about a substantial change in the boiling point of thenaphthene hydrocarbons and subsequently separating these isomers byordinary fractional distillation. In this regard various isomerizationmechanisms may be employed. Mixtures or fractions containingpolyalkylated naphthenes for example may be separated by isolation of aspecific fraction containing only one of the geometrical isomers of thepolyalkylated naphthene, then subjecting this fraction to anisomerization reaction thereby converting the particular isomer toanother geometrical form having a boiling point appreciably differentfrom that of the fraction in which it is contained and subsequentlyfractionating the isomerized product to separate the isomers from theoriginal fraction.

For example in the 'separation of dimethylcyclohexanes ordimethyleyclopentanes from a petroleum fraction, the fraction issubjected to a controlled distillation to isolate that portion of thefraction which contains the lower boiling or trans isomers and thatportion of the fraction which contains the higher boiling or cisisomers. These narrow boiling cuts are subsequently isomerized toconvert the trans isomers in the one case to the cis isomers, and thecis isomers in the other case to the trans isomers. Such anisomerization elfects a change in boiling point of the naphthenes ofabout 5 C. to about 8 C. and permits their 3 separation from therespective fractions by controlled distillation.

In a similar manner other isomerzation mechanisms may be utilized toeffect the change in the boiling point of the naphthenic hydrocarbons inany petroleum fraction, such isomerization reactions including:

1. Change of the relative position of the alkyl group in the naphthenering.

Change in the naphthene ring structure; that is, conversion of a C5 ringto a Cs ring, or of a Cs ring to a C5 ring and the like.

3. Isomerization of the parafnic side chain attached to the naphthenegroup such as conversion of a propyl group to an isopropyl group and thelike.

4. Disproportionation of the alkyl groups in the side chains such asconversion of two methyl groups to one ethyl group or conversion of twoethyl groups to a methyl group and a propyl group and the like.

We have found that by effecting an isomerization reaction of one of thetypes described we are able to effect the separation of any naphthenefrom a narrow boiling hydrocarbon mixture in which it is contained. Itis understood that the thermodynamic equilibrium in these cases is notcomplete and as a result only partial conversion to an isomeric form ispossible in a one step operation. As a result it may be necessary toemploy two or more stages or a continuous isomerization to effect thesubstantially complete removal of the naphthenic hydrocarbons by ourprocess.

It is within the scope of our invention to effect the isomerization ofthe naphthenes in the presence of any desirable isomerization catalyst.We have found that such metallic catalysts as iron, copper, platinum,palladium, reduced nickel, Raney nickel, cobalt and others havinghydrogenating activity; metallic oxides, such as alumina, titenia,chormie, zinc oxides, molybdenum oxide or other oxides havingdehydrogenating activity; acid materials such as phosphoric acid, acidphosphates, sulfuric acid, acid sulfates, acid halides such as aluminumchloride or bromide, acid suldes such as cobalt sulfide, molybdenumsulfide and the like are effective catalysts for our process. All ofthese catalysts may be employed in the pure state or in combination withvarious activators or carriers. Generally speaking the aluminum halideswill cause isomerization under the mildest condition, that is at thelowest temperatures, while the acidic isomerization catalysts,hydrogenation catalysts and the dehydrogenation catalysts will requirehigher temperatures for the isomerization in the order given.

When typical dehydrogenation or hydrogenation catalysts are used thetemperatures to be employed will range from room temperatures up toabout 300 C. depending upon the nature and activity of the catalyst,although higher temperatures may be employed if desired.

As the isomerization reaction is an equilibrium reaction in which theequilibrium concentration of the reactants is dependent on thetemperature, the isomerization has to be carried out at such atemperature as to obtain favorable conversion. Thus, if the desiredreaction is geometrical isomerization we have found that the transisomers, the lower boiling isomers, are the stable form at lowertemperatures and isomerization of cis isomers to the trans isomersshould be carried out at the lowest practical temperature. Conversely athigher temperatures the cis-trans equilibrium is shifted towards theless stable cis isomers and thus in isomerization of a fraction rich inthe trans isomer higher isomerization temperatures will favor theformation of the cis isomer.

We have found that hydrogenation or dehydrogenation catalysts will notcause appreciable dehydrogenation of the -membered ring naphthenehydrocarbons such as for example the cyclopentane homologs and lowernaphthenes at the temperatures required for carrying out theisomerization. On the other hand dehydrogenation may occur whenemploying catalysts of these types in the isomerization of theG-membered ring naphthene hydrocarbons such as cyclohexane, its homologsand the higher naphthenes. In order to repress any undesirabledehydrogenation of these latter naphthenes onhydrogenation-dehydrogenation catalysts the isomerization is carried outat the lowest possible temperature in the presence of suicient hydrogento shift the hydrogenation-dehydrogenation equilibrium between thenaphthene homologs and aromatic homologs in favor of the naphthenehomologs. Alsofwe have found that the presence of a small amount ofhydrogen in carrying out the isomerization of the -membered ringnaphthenes on hydrogenation or dehydrogenation catalysts has a favorableeffect even though in this case its role is not necessarily theprevention of dehydrogenation.

The process of our invention may be better understood by reference tothe accompanying drawing which exemplifies one modification of theseparation procedure 0f our invention. For the sake of clarity thedrawing will be described with reference to the separation of puredimethylcyclopentane from a petroleum hydrocarbon fraction but we do notin any way mean to limit our invention to this separation or this methodof procedure.

The majority of petroleum stocks contain dimethylcyclopentanes, thetrans isomers (boiling point 90.7 C. and 91.8 C.) being present in mostcases in larger proportions than are the cis isomers (boiling point 97C. and 99.2 C.) However, these isomers are both present to a certainextent and their separation from the petroleum fractions are renderedvirtually impossible by the presence of such parain hydrocarbons as 2-methylhexane, (boiling point 89.7 C.) S-methylhexane, (boiling point91.8 C.) S-ethylpentane, (boiling point 93.3 C.) n-heptane (boilingpoint 98.4 C.) and 2,2,4-trimethylpentane (boiling point 99.3 C.).Although all of these parafiins may not be present in all petroleumstocks the presence therein of any one or combination of these willrender the preparation of substantially pure dimethylcyclopentanesvirtually impossible by ordinary fractional distillation.

In employing our process a fraction boiling between about 89 C. andabout 100 C. is employed and with reference to the drawing such afraction is withdrawn from the hydrocarbon feed tank I0 through line I Icontrolledbyvalve I2 andpumped by pump 3 via line I 4 into fractionatingcolumn I6. In fractionating column I6 conditions are so controlled as togive an overhead fraction boiling between the initial boiling point ofthe feed or about 89 C. and about 92 C. to 93 C. This fractioncontaining substantially all of the trans isomers of1,2-dimethylcyclopentane and 1,3-dimethyl-v cyclopentane which werepresent in the hydrocarbon feed, and also such paraffin hydrocarbons as2-methylhexane, S-methylhexane, and 3- ethylpentane is taken overheadvia. linel I1 con trolled by vaslfve t8. passing therefrom via line I9'into condenser 20 from which it drawn into collecting tank 2 I. Thecondensate is. withdrawn from condensing tank 2l and is pumped by pump22 into line 23 controlled by valvesy 24 and 25 and a. part thereof maybe returned via line 25 to fractionating column t6 to serve as refluxlthe distillation, the remaining portion being passed Vial'ne 2T throughheater 28 and' line 2.9- into reactor 30. In one modication of ourinvention as exemplified by the drawing, reactor 3D is divided into twoportions, the'l upper pore tion of which consists of an isomerizationcata@ l'yst, which maybe any desired catalyst', and the lower portioncontains packing or trays as de# In reactor 3B a temperature'equilibrium is maintained so that theffeed boiling between about 89 andabout 93 C". is' maintained in the upper or catalysty portion of thereactor. isomer'- ization takes place therein converting the transisomer of dimethylcyclopentane to the higherboiling cis isomers whichupon formation, because of their higher boiling point, drop to the'bottom portion of the reactor and may be withdrawn theree fromcontinuoushr through line 3|` controlled by valve 32 and pumped by pump33 vialine 34 to Storagetank 35. By operation of reactor 39 in this manerier` the equilibrium concentrations of the cis and trans isomers isbeing continually destroyed by removal of the cis isomers as they areformed and substantially complete isomerioation of the trans isomers maybe had. The overhead fraction froni` reactor illicornprisingsubstantially naphthene free hydrocarbons is withdrawn via' line 36`controlled by valve 31 passing therefrom through lin-e 38- intocondenser 40 the condensate being collected in collecting tank 4f. Thesub-Y stantlally naphthene free lcondensate is withdrawn from collectingtank 4i, is' pumped by pumpv 42 into line 43', a part thereof may be reAturned through valve 4'4- and line 4'5 to the cata@ lyst section of the'reactor 30' in order to further insure substantially completeisomerizati'on of the trans dimethylcyclopentanes in the fraction'. Theremainder of the condensate passesvfrom` line 43 through valve 6G andlines 4T and 48 into the gasoline-storage tank 50.

Returning to fractionating column Hi a bot-, toms productV is obtainedcomprising that part of the initial feed boiling above about 93 C.,this' product isy withdrawn from column I't by line 5l controlled byAvalve 52 and is pumped by'npump 53' via line 54" into fractionating4column titi. In fractionating column 5'64 an essentially naphthene freeheart cut boiling between about 93 C. and about 96 C. or 97" C. isobtained as an overhead product and is withdrawn from the column throughline 5l, controlled by valve 58 through I line 59 into condenser 6U fromwhich the corid'ensate passes to collecting tank 6l'. The con-A densateis withdrawn from collecting tank 61 and is pumped by pump E2 into line453' a part thereof may be returned through valve 64' and line166 to'fractionating columnv vSii to serve as reflux in the distillation, theremainder passing through valve 6-5 lines 61 and 4'8 to the gasolinestorage tank' 5U..

The bottoms; fraction from the fractionating column comprising afraction boiling in the. range of about 97 C; to about 100' C. andcontainingsubstantially all of cis isomers of LZ-dimethylcyclopentaneand 1`,3-dimethylcyclopen tane present in the initial hydrocarbon feed,is withdrawn rr'om column `5t through line 6% con-l.' trolled by valve69I and is by pump 1'0" through liner 'H into heater 12 passingvtherefrom through line. lf3 into reactor 14. Because of the nature ofthe isomerization reaction in this case, reactor 14 represents thedirect opposite of reactor 3'0. The isomerization of the cis isomersresulting in the formation of the lower boiling trans isomersnecessitates the presence of a catalyst the lower portion of reactor 14.In this manner the unconverted cis isomers remain in contactV thecatalyst in the lower portion of the reactor while the trans isomers,which are formed the reaction pass upward therefrom intoy thefractionati-ng. section of the reactor con-JV taining packing orconventional trays and are taken overhead therefrom. These reactors areconstructed in this manner to prevent a further contact of the desiredisomers, formed in the reaction., with. the catalyst inasmuch as suchcontact would lead to they undesirable isomerization ofi these isomers;back to the isomers as contained` in the feed to thev reactor andprevent the continued isomerization of the naphthene in the feed Thesection of the reactor not containing catalyst serves as: a,fractionating column and separates the isomers formed from otherhydrocarbons which may be carried along therewith from the. catalystsection of the reactor causing said hydror'zarbonV of different boilingpoint to be returned to the catalyst section and permittingv the removalfrom ther reactor of the pure iso-S meric dim'ethylcyclopen'taries.vFrom reactor 'N' the trans dimethylcyclopentanes are obtained asoverhead products and are withdrawn from the reactor via.- line T5,controlled by valve 1B pass'-` ing through lin'e 'l1v into condenser 18passing therefrom to collecting tank 19; The condensate is withdrawn.from collecting tarrk 19 and pumped by pump' 80 into line 8l'- and apart thereof may be returnedby valver 82 through line 84 into thereactor 'I4 to serve as reflux in the distillation taking piace in theupper part of reactor 14. The remainder of the 'condensate passes fromline 8i through valve 83v and line 85 into the storage tank 855.l Thebottoms product from column 'M comprising essentially naphthene freehydrocarbon isy withdrawn from reactor 14' via line 81 controlled b-yvalve 88 and is pumped by Dump 89 through line into the gasoline storagetank 50.

In this manner we are able to obtain from a narrow boiling petroleumfraction containing4 the Various isomeric. formsl ofdimethylcyclopentane essentially paraflin free dimethylcyclopen tane andessentially naphthene free paraiiins. Providing; the hydrocarbon feed isa straight ruri' feed we: are this manner able to obtain notonlysubstantially pure` naphthenes but also sub stantiall-y pure paraflins.Such a process as de a scribed isd ecrually applicable tothe separationof other `napl'ithencs'. such as for example dimethyl-v cyclohexaries;Further, it is desired to sepa rate naphthenic hydrocarbons which do notoccur in both cis and trans" forme, that is by an isomerization reactionin which -geom'ertic isomerica tion is. not involved the process' may besimilar to only one stage in the above description, that is the fractioncontaining the naphthenic hydro'-v carbon may be fed directly to acolumn suchy asA column 30 or Mr as described above in which the:

catalyst occupies one half of the reactor and thev frfactionatingl zone.occupies the other half, the placements of these two being dependentsolely on the type of isomer formed from the naphthener i'r thehydrocarbon feed. Thus, if a lower boilingA isomer is formedv thereaction the; catalyst, section should bein the lower part of thereactor and the fractionating section in the upper and conversely if theisomer is higher boiling the catalyst should Ibe in the upper section ofthe reactor and the packing in the lower section. As pointed out abovethe catalyst placement is made with the idea in mind of preventing anysubsequent contact between the catalyst and the isomerized naphthene andfurther to prevent the establishment of an equilibrium therein and theconsequent cessation of the isomerization reaction.

The process as described is only illustrative of our invention inasmuchas many procedural modifcations are possible and will occur to thoseskilled in the art. For example a single catalytic reactor may beemployed through which the entire hydrocarbon feed is passed to effectthe partial isomerization of the naphthene contained in the fractionwhich reactor may be followed by a distillation column to effect theremoval of the isomerized product and a portion of the unisomerizedproduct may be recycled to the reactor.

Examples of other isomerization reactions may further clarify theprocess of our invention. In addition to the geometric isomerization asdescribed above we may bring about a change in the relative positions ofthe alkyl group. Thus, if a hydrocarbon mixture containing1,4-dimethylcyclohexane trans (boiling point 119.3 C.) methylheptanes(boiling point 117.6 to 118.9 C.) 3-ethyl hexane, (boiling point 118.5C.) 3,4- dimethylhexane, (boiling point 117.7 C.) is isomerized in thepresence of aluminum chloride at 50 C. to 100 C. the1,4-dimethylcyclohexane is partially converted to trans- 1,3 and1,2-dimethylcyclohexane, (boiling point 120.7 C. to 123.4 C.) and to1,4-, 1,3- and 1,2-dimethylcyclohexane-cis (boiling point 124.3 C. to129.8 C.). These isomerized dimethylcyclohexanes are readily separableby distillation from the parain hydrocarbons which are essentiallyunchanged.

Further it is possible to effect the naphtheneparamn separation by theisomerization of the napthene hydrocarbon in such a manner as to changethe ring configuration thereof. For example if a mixture of cyclohexane(boiling point 80.7 C.) and 2,4-dimethylpentane, (boiling point 80.5 C.)is treated with aluminum chloride and anhydrous hydrogen chloride at theboiling point of the mixture the cyclohexane will isomerize tomethylcyclopentane (boiling point 71.8 C.) and may be readily separatedfrom the mixture by distillation. In such a case as this wherein a lowerboiling naphthenic isomer is formed in the isomerization it is necessaryin a continuous process, as pointed out above, to place the catalyst inthe lower portion of the reactor and the packing or trays in the upperportion of the reactor in order to minimize any continued contactbetween the catalyst and the naphthene isomer as it is formed, the lowerboiling isomer passing upward in the column into the distillationsection. The description of reactor '14 of the accompanying drawingexemplies this type of operation. Cyclohexane may be converted tomethylcyclopentane by numerous other catalysts as for example molybdenumsulfide-cobalt sulde at 500 C. Other naphthenic hydrocarbons aresusceptible to what may be classified as ring isomerization and We donot Wish to be limited by the example above. For examplemethylcyclohexane may be isomerized to dimethylcyclopentane overmolybdenum sulde-cobalt sulfide catalyst at 500 C. to effect a reductionof the boiling point and permit the separation thereof from paransboiling in the neighborhood of C. by distillation. In operations of thistype it is apparent that those isomerzations in which the number ofcarbon atoms in the ring structure are increased will result in a higherboiling naphthene isomer, and conversely those in which the number ofnuclear carbon atoms is decreased will result in an isomer of lowerboiling point. It is therefore necessary to control the variousoperational factors such as catalyst placement, type of distillation andthe like accordingly.

Another type of isomerization which may be utilized according to theprocess of our invention involves the disproportionation of the alkylgroups in the side chains of naphthenic hydrocarbons. Thusethylcyclopentane (boiling point 103.4 C.) may be separated from2,2-dimethylhexane (boiling point 107 C.) by subjecting the mixture tothe isomerizing action of aluminum chloride or other catalysts wherebythe ethylcyclopentane is isomerized to dimethylcyclopentanes (boilingpoint 91 C. to 92 C.) which dimethylcyclopentanes may be readilyseparated from the dimethylhexane by ordinary distillation. In similarmanner propylcyclohexane (boiling point 155 C.) may be isomerized overaluminum chloride at C. to 145 C. to yield 1,3,5-trimethylcyclohexane(boiling point C.) representing a drop in boiling point of 15 degreespermitting ready separation from paran hydrocarbons in the originalmixture by distillation.

In many cases it may be desired to recover a specic naphthenehydrocarbon or hydrocarbon group. If such be the case there aredifferent methods of operation by which the desired end may beaccomplished. The specific naphthene or group of naphthenes may beobtained by selecting a petroleum fraction containing a naphthenedifferent from the desired naphthene which may readily be isomerized tosuch naphthene and separated as such from the petroleum fraction. Thusfor example if substantially pure dimethylcyclopentanes are the desiredproduct We may isolate a narrow boiling hydrocarbon fraction rich inmethylcyclohexane, subject such fraction to an isomerzation convertingthe methylcyclohexane to dimethylcyclopentanes which are subsequentlyrecovered from the mixture by distillation. In a similar manner otherpure naphthenes may be obtained such as for example methylcyclopentaneby the isomerization of cyclohexane rich fraction,1,2-dimethylcyclohexane from the isomerization of a hydrocarbon mixturerich in ethylcyclohexane, dlmethylcyclohexanes from propylcyclopentaneand the like.

For another method of operation to obtain specific naphthenehydrocarbons, we may utilize a fraction containing the naphthene and byisomerizing this naphthene convert it to an isomer of suflcientlydifferent boiling point to permit its separation from the originalfraction by distillation, and subsequently subject the separated isomerto a second isomerization at conditions which favor the reverseequilibrium to convert it back to the naphthene sought. For example inorder to obtain pure trans-dimethylcyclopentanes we may employ ahydrocarbon fraction rich in trans-dimethylcyclopentanes, subject thisfraction to an isomerization wherein the trans isomers are isomerized tothe cis forms, separating the cis-dimethylcyclopentanes and subjectingthese to an isomerization to effect a return to the trans form. In suchan instance the isomerizing conditions are chosen in each case to favorthe equilibrium in regards the isomeric form sought. Such an operationmay be conveniently carried out continuously ,by a method similar tothat described above. Such a secondary or reverse isomerization may beemployed to obtain specic naphthenes whenever the equilibriumconcentration of the isomerc forms may be suitably adjusted by changesin temperature, pressure, contact time or the like.

Having described our invention and realizing that many modificationsthereof may occur to those skilled in the art without departing from theunderlying principles set forth for the recovery of pure naphthenichydrocarbons from complex hydrocarbon mixtures, we claim:

1. A process for separating substantially pure dialkylated naphthenesfrom a hydrocarbon mixture containing dialkylated naphthenes and paramnhydrocarbons boiling in the same temperature range as the dialkylatednaphthenes which mixture is diiiicult to separate by means of ordinarydistillation because of the closeness of the boiling points of thecomponents in said mixture, the process comprising fractionating themixture to obtain a narrow boiling fraction containing essentially onlythe cis type of geometric isomer of said dialkylated naphthenichydrocarbon together with parain hydrocarbons and a second narrowboiling fraction containing essentially only the trans type of geometricisomer of said dialkylated naphthenic hydrocarbon together with paraiiinhydrocarbons, isomerizing one of said narrow boiling fractions toconvert the naphthenes contained therein from said geometric isomer tothe other geometric isomer thereby effecting a change in the boilingpoint of said naphthenes, and distilling the isomerized product toseparate the geometric isomer formed in the isomerization from theparaiin hydrocarbons in the mixture.

2. A process according to claim 1 in which said dialkylated naphthenesare dimethylcyclopentanes.

3. A process according to claim 1 in which said dialkylated naphthenesare dimethylcyclohexanes.

4. A process for separating substantially pure dimethylcyclopentanesfrom a hydrocarbon mixture containing dimethylcyclopentanes and parailinhydrocarbons boiling in the same temperature range as saiddimethylcyclopentanes, which dimethylcyclopentanes are difcult toseparate by means of -ordinary fractional distillation because of thecloseness of the boiling points of said dimethylcyclopentanes with theboiling point of parain hydrocarbons contained in said mixture whichcomprises fractionating said mixture to obtain a narrow boiling fractionrich in the trans isomers of dimethylcyclopentane and the secondfraction rich in the cis isomers of the dimethylcyclopentane,isomerizing each of these fractions separately to effect theiscmerization of the trans isomers in the one fraction to the ciis-omersand the cis isomers in the other fraction to the trans isomers therebyeffecting a change in the boiling points of the dimethylcyclopentanes ineach fraction, and separately distilling the products from theisomerizations to remove therefrom the isomerized dimethylcyclopentanes.

5. A process for separating substantially pure dimethylcyclohexanes froma hydrocarbon mixture containing dimethylcyclohexanes and paraiiinhydrocarbons boiling in the same temperature range as saiddimethylcyclohexanes, which dimethylcyclohexanes are difficult toseparate by means of ordinary fractional distillation because of thecloseness of the boiling points of said dimethylcyclohexanes with theboiling point of parafn hydrocarbons contained in said mixture whichcomprises fractionating said mixture to obtain a narrow boiling fractionrich in the trans isomers of dimethylcyclohexane and the second fractionrich in the cis isomers of the dimethylcyclohexane, isomerizing each ofthese fractions separately to effect the isomerization of the transisomers in the one fraction to the cis isomers and the cis isomers int-he other fraction to the trans isomers thereby effecting a change inthe boiling points of the dimethylcyclohexanes in each fraction, andseparately distilling the products from the isomerizations to removetherefrom the isomerized dimethylcyclohexanes.

ADALBERT FARKAS. ARTHUR F. STRIBLEY, J R.

REFERENCES CITED The follorwing referenlces are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Egloi et al.: Isomerization ofPure Hydrocarbons, A. C. S., Monograph Series, No. 88 (1942), ReinholdPub. Co. (N. Y. City), pages 91 to 107.

